In typical digital oscilloscopes of the prior art, the signal being monitored is actually sampled by the oscilloscope during a very small percentage of the time that the scope is presenting the display to the user. During the balance of the time, the scope is "blind" to the signal activity. At best, this can be frustrating to a user who is trying to see an intermittent problem. In the worst case, the user is unaware of this limitation and erroneously thinks that the signal is being monitored for a much higher percentage of the time than is actually the case.
In typical analog oscilloscopes of the prior art the voltage versus time behavior of the signal being observed is presented in real time on a cathode ray tube (CRT) display. An electron beam is moved horizontally across the display at a constant rate that is determined by a timebase setting. As the electron beam moves horizontally at this constant rate, the time-varying voltage level of the signal being observed controls the vertical position of the electron beam.
Even though the electron beam may be moving far too quickly to be perceived by the human eye, repetitive signals can still be perceived because of the persistence that is inherent in the light emitted by the phosphor coating of the CRT. Typically, for a repetitive signal to be visible to a human observer, the sweep across the CRT must be repeated at many times per second, with the exact number depending on other factors such as the beam intensity. The actual sweep speed can be much faster, e.g., 10,000 or more updates per second. Depending on how much or how little a "trigger holdoff" control is applied, the signal being monitored may actually be visible on the face of the CRT up to 90% of the time or more.
The analog system just described has, however, one major limitation which is important to the present discussion, i.e., that rare, anomalous, non-repetitive events will usually go completely undetected, since by definition they are not repetitive enough to appear on the display as often as is necessary for perception by the human eye. To compensate for this limitation, the display can be enhanced by the use of an electron multiplying faceplate, such as the microchannel plate system described in U.S. Pat. No. 4,752,714 to Sonneborn et al. for "Decelerating and Scan Expansion Lens System for Electron Discharge Tube Incorporating a Microchannel Plate" and U.S. Pat. No. 5,134,337 to Kongslie et al. for a "Projection Lens Assembly for Planar Electron Source", both of which are hereby incorporated by reference. An analog oscilloscope having a display enhanced by this microchannel plate technology can amplify a rare event to make it visible, so that such an event remains perceptible to the human eye for more than a second after only a single occurrence.
Unfortunately, microchannel plate technology is relatively expensive and, because of the high beam intensities that it generates, it is also prone to causing damage to the CRT phosphor unless the CRT is protected from over-exposure to the beam. When the intensity of such a system is turned up to view a rarely occurring signal anomaly, protective circuitry designed to avoid CRT burning will automatically reduce the intensity to avoid damage. This automatic dimming during operation creates a tension between the operator's desires and the display system's limitations, and this can be irritating and frustrating to the user. And, since this is an analog system in which the signal is displayed but not digitized and stored, it is not possible to store a waveform for later viewing.
In digital oscilloscopes the signal whose behavior is being monitored is sampled at regular intervals and each of these samples is quantized as a digital number that can be stored and otherwise processed before it is displayed. Typically, incoming analog waveform data is quantized into numerical values by an analog-to-digital converter at regular intervals determined by an acquisition clock signal. These numerical values are stored in acquisition memory at locations corresponding to successive time increments. A waveform processor performs any desired manipulations of this data, and stores the results in a display memory. A display controller then accesses the contents of the display memory and presents the resulting waveform on a display.
In a first type of digital oscilloscope, the quantized sample values are processed as desired and then converted back to analog voltages for display on a conventional CRT. In this type of system the maximum display update rate is typically about 50 to 60 times per second because considerable processing and display time is associated with each display cycle. If the sweep speed of such an oscilloscope corresponds to 10,000 records per second, but only 50 or 60 of these potential records are actually processed and displayed, that means that less than one percent of the signal's behavior is available for viewing by the operator and more than 99% is lost from view. Such a characteristic seriously detracts from any possibility of finding an intermittent event of interest.
In a second type of display system for digital oscilloscopes, the display is stored in a digital bit map and presented on a raster scanning CRT display without ever being converted back into an analog signal. In this type of system the maximum display update rate is about 70 times per second because rasterization is typically performed by software and a microprocessor, and this requires that a lot of time be devoted to processing the contents of each display. Thus, for sweep speeds corresponding to 10,000 records per second, less than one percent of the signal's actual activity is available for viewing by the oscilloscope operator, so the chances of finding random anomalous signal behaviors is very small and when such behaviors are captured they are not visible to the human eye unless they happen to be stored and held for non-realtime viewing. The bit map type display can be made to behave more like a conventional analog CRT type display by causing the contents of the bit map to decay over time as newly acquired signal traces are added to it.
Once a rasterized image is created, it can be displayed indefinitely, which is sometimes known as "infinite persistence", or it can be caused to decay over time to emulate the normal persistence of the phosphor CRT screens used in analog oscilloscopes. U.S. Pat. No. 5,254,983 to Long et al. for "Digitally Synthesized Gray Scale for Raster Scan Oscilloscope Displays", hereby incorporated by reference, discloses a method for digitally creating the effect of persistence, i.e., intensity that diminishes as a function of time. Pending applications by Alappat et al. having Ser. Nos. 07/149,792 and 07/563,656 disclose similar and related techniques for creating digital persistence effects.
It would be highly desirable to have a digital oscilloscope that allowed a user to observe an input signal for a much higher percentage of the time than has heretofore been available, and that allowed a user to reliably see input signal anomalies even when they occur only intermittently. Having these capabilities in a digital oscilloscope is important for several reasons. Digital oscilloscopes allow storage of acquired waveforms, have relatively unlimited record lengths, and permit the acquisition of information that occurs before a triggering event. They also have superior display accuracy because a raster-based display is not subject to CRT errors.
One step in the desired direction has been taken in the design of the Hewlett-Packard 54600 Series digital oscilloscopes. As is described in an article in the February 1992 Hewlett-Packard Journal entitled "A High-Throughput Acquisition Architecture for a 100-MHz Digitizing Oscilloscope", these digital oscilloscopes were designed to seem more like analog oscilloscopes to their users. To achieve this, rasterization in software was replaced by rasterization in dedicated hardware. However, the total throughput of this system is limited by the fact that the raster image memory (video RAM) is used both to provide the raster image to the display and to receive the output of the rasterization hardware. This dual use of the raster image memory means that it cannot achieve the throughput possible with the invention to be described below.